This invention relates to a method of removing sulfur dioxide from flue gases and oxidizing calcium sulfite in the discharge from a flue gas desulfurization (FGD) process.
In some commercial wet FGD processes, sulfur dioxide-containing flue gases are contacted with aqueous slurries containing mainly calcium hydroxide, and optionally up to about six weight percent magnesium hydroxide, in a scrubbing tower where the sulfur dioxide is removed from the gases as calcium sulfite and calcium sulfate in the slurries. In some processes, slurries are recirculated around the scrubbing tower with at least a minor portion of the slurries being first diverted to an oxidation step. In this oxidation step, the calcium sulfite and calcium hydrogen sulfite are oxidized to form additional calcium sulfate in an aqueous solution. The calcium sulfate is separated from the aqueous solution and recovered as gypsum and the aqueous solution is recirculated to the scrubbing tower. The solution recirculated from the oxidation step usually contains sulfate ions which react with calcium from calcium hydroxide in the scrubbing tower to form additional calcium sulfate. U.S. Pat. Nos. 2,080,779 and 4,046,856 generally disclose such wet FGD processes.
As the technical and patent literature indicate, calcium sulfite and calcium sulfate processes and products are very difficult to handle because of their rheological properties, and they tend to clog and corrode the equipment and interconnecting piping. Thus the close integration of the scrubbing and oxidation steps complicates the efficient operation of the scrubbing towers. In addition, poor quality calcium sulfate crystals are formed under the general operating conditions in scrubbing towers.
In a modified commercial FGD process designed to facilitate the operation of the scrubbing tower, the scrubber slurry from the scrubbing tower is first separated in a thickener to remove insoluble calcium sulfite and other solids before it is recirculated. The thickener underflow is then discharged as a waste product in the form of a sludge or filter cake. This waste product raises environmental concerns. Thus it has been suggested that the waste product might be converted to a marketable gypsum by-product by oxidizing the calcium sulfite to calcium sulfate.
It is an object of the present invention to provide an improved method for removal of sulfur dioxide from flue gases where discharged sludge is oxidized to produce marketable gypsum.